It is well known that many kinds of vegetable material in varying amounts contain oil, i.e., liquid triglycerides, which can be extracted as a valuable commodity, and numerous processes aimed at such extraction have been proposed. Many vegetable seeds contain a significant quantity of oil, such as soybeans, castor beans and cottonseeds. In some instances the oil is essentially present in a specific constituent, such as the germ or a covering layer or sheath known as bran, as in the cereals, the bran of rice being especially high in oil. This covering or bran is removed during the initial milling operations of the cereal grains as, for instance, in the case of rice, in the conversion of so-called brown rice to white or polished rice, the exterior brown layer thus-removed being recovered as so-called rice bran. Rice bran oil has for many years been extracted from rice bran and is valued as a cooking or frying oil, particularly in Oriental cultures. The oil-free or defatted bran residue resulting from such extraction has up to now possessed only limited value as a commodity but is presently being evaluated as a desirable bulk or natural fiber material for human ingestion with possible medicinal benefits in reducing serum cholesterol levels.
Of the many extraction processes heretofore proposed, most employ a normally liquid solvent for the oil, of which hexane is the most common example. Hexane is a liquid at ambient temperature and pressure so that the extraction can be carried out without pressurization at ambient or higher temperature. With the extraction vessel under normal atmospheric pressure, the introduction and removal of the raw vegetable matter into the extraction vessel can be readily carried out in a continuous manner, and the following prior art patents are representative of known continuous extraction techniques utilizing the hexane or similar hydrocarbons as the extraction solvent.
According to U.S. Pat. No. 2,206,595, oil-containing raw material is extracted in a vertical column, to which it is delivered from storage via a feed screw, the column being divided into multiple stages separated by rotating perforated discs, each associated with a fixed scraper blade, the extraction solvent, e.g., hexane, being delivered to the bottom of the column with the mixture of dissolved oil and solvent, called miscella, being removed near its top. In communication with the bottom of the column via a feed screw and feed box is a vertically extending conveyor of the flight type moving in a continuous generally L-shaped path, which carries an ascending column of the extracted solid residue in admixture with solvent. The conveyor extends above the working height of the extraction column and, being filled with solvent, functions as a barometric leg offsetting the hydrostatic pressure of the extraction medium in the column. Miscella is drawn off below the top of the conveyor leg with the solid residue discharged at a higher point on the leg.
In a later filed U.S. Pat. No. 2,203,666 the same inventor disclosed an alternative way of supporting the hydrostatic head of the vertical extraction column in the form of a helical screw conveyor or packer screw receiving the solvent-solid residue mixture delivered by a superposed feed screw of smaller diameter out of the column bottom and advancing the same through an annular passage having its central core flaring somewhat outwardly to narrow the annular radius thereof. A spring biased conical disc pressed against the discharge end of the annular passage, applying mechanical pressure to the solid matter there so as to create a compressed "plug" of the solid residue of sufficient density as to support the hydrostatic head of the extraction medium.
Earlier in U.S. Pat. No. 2,184,248 Bonotto had combined the feed screw and packing screw coaxially, the larger packing screw receiving incremental amounts of solid residue packing the same against a spring loaded yieldable conically shaped closure disc into a solid plug impervious to the flow of solvent therethrough.
More recently, in Schumacher U.S. Pat. No. 4,390,506 an extraction vessel is fed a slurry of recovered miscella and raw material under conditions minimizing exposure of the raw material to atmospheric oxygen rapidly causing rancidity. Thus, the flaked raw material discharges directly into a vertical hopper maintained under a vacuum aspirating entrained air. Near the hopper bottom recycled miscella is admitted to form a slurry, and the miscella-raw material slurry gravitates into the lower end of an upwardly inclined screw conveyor which dumps at its upper end into the extraction chamber. The hopper-inclined conveyor arrangement is kept filled with miscella, thereby continuously immersing the raw material in miscella and precluding contact with air. Fresh solvent is fed at the top of the extraction chamber, and the miscella is taken from the bottom thereof, a part being recycled to the raw material hopper and the remainder recovered for use. Solid residue is also taken from the bottom of the extraction vessel by a screw conveyor.
A mechanically simplified and refined system was proposed by Schumacher in U.S. Pat. No. 4,617,177 using the screw conveyor itself as the extraction chamber, the screw conveyor being horizontally disposed with the pitch of its flights increased or widened toward its discharge end. The raw material and extraction medium, i.e., solvent and/or miscella, are both introduced into the end of the conveyor where the flights are closest and additional medium is also supplied at axially spaced points along preferably about the first half of the conveyor length. Extracted solid residue is discharged from the opposite end of the conveyor, and just upstream of that end the conveyor is perforated for passage of the miscella. Several such units can be combined in series, the miscella from each such stage is perferably collected separately and recycled to the immediately preceding stage. Effecting extraction contact directly in a screw conveyor of expanding pitch achieves efficient mixing, avoiding the considerable excess of such medium normally required for complete wetting in extraction columns.
Although the extraction conditions and equipment for oil-containing vegetable material using a normally liquid solvent, e.g., hexane, are technologically simple, these advantages are considerably offset by the problems encountered in separating the solvent from the extracted oil and defatted meal. Separation of the normally liquid solvent from the oil involves a distillation operation, requiring the application of large amounts of heat to the miscella, and, moreover, the essential elimination of the solvent from the recovered oil by distillation is difficult, if not impossible, because the recovered oil has a preferential attraction to a certain minimum amount of solvent. Similarly, complete removal of solvent such as hexane from the solid residue is virtually impossible by practical desolventizing techniques such as steam-stripping or the like. By way of illustration, analysis of hexane loss in a modern rapeseed extraction facility with a capacity of 800 MT/day revealed the presence in the meal of 0.18% by weight of hexane, despite the treatment of the meal in a desolventizer-toaster.
The presence of such levels of residual hexane in the recovered oil and/or solids residue is undesirable and is in fact prohibited by government regulation in some localities. Disregarding the possible deleterious consequences of human consumption of such products, even this apparently small quantity of solvent represents an appreciable loss if solvent in economic terms based on large volume production. Thus, the facility described above experienced a hexane loss in the meal alone of well in excess of 1 MT/day.
For more than fifty years, the art has recognized that such oil-containing vegetable material can also be extracted with normally gaseous hydrocarbon solvents, such as propane, by imposing during extraction a sufficient pressure in the extraction chamber to maintain the solvent in liquified condition. Operation pressures proposed in such process vary widely, but as a minimum must exceed about 125 psi for propane at room temperature operation and even higher pressures if the temperature is elevated. However, the introduction of the raw vegetable matter into the extraction chamber, as well as the removal of the solid residue therefrom is complicated at such levels of pressure, and initial proposals utilizing this approach involved batch operation, usually with two or more extraction vessels being arranged in parallel for alternative onstream operation so that one could be filled or emptied, while the active extraction is being carried out in the other.
Early examples of batch-type extraction processes utilizing a pressurized liquified gas, such as propane or butane, are disclosed in U.S. Pat. Nos. 1,802,533 and 1,849,886. In the former, an extraction vessel is filled with raw material to which the liquified solvent is supplied, and after extraction the miscella or solvent extracted oil is distilled by heating, driving off the solvent vapors which are condensed and recycled for subsequent extraction, the oil remaining after the distillation for removal and use. In the latter, a plurality of extraction units, e.g., five, are arranged in parallel and the separation and recovery of the solvent are achieved in more efficient fashion through the assistance of vacuum. Vacuum may be applied to the extraction chamber after loading with raw material to exhaust the air therefrom before admission of the liquified solvent as well as to the extracted oil remaining after heat distillation to maximize elimination of solvent. The distilled solvent vapors are fractionated in a fractionation column and after condensation recycled to solvent storage for reuse.
Although the use of normally gaseous extraction solvent in pressure liquified condition requires elevated pressures, this disadvantage is to some extent offset by the more ready removal of the solvent from both the solids residue and the extracted oil due to its inherent vaporization when the liquifaction pressure is removed. Nonetheless, efficient recovery of such solvents calls for rather complicated recovery systems such as that seen in U.S. Pat. No. 2,560,935. Using two extraction vessels, operating batchwise in alteration, the extraction mixture, consisting of solid residue, extracted oil and solvent, is here delivered to a settling tank wherein the heavy solid residue settles out for passage to a separator vessel from which the entrained solvent is allowed to escape under reduced pressure. The miscella is taken from the top of the settling tank and is transferred to a phase separation tower with heating either by a preheater and/or tower heaters to a temperature near the critical temperature of the solvent and above the maximum solubility temperature of the oil dissolved therein. Consequently a more dense liquid oil phase separates and entrains solid fines therein for discharge from the tower bottom. The remaining oil-solvent mixture is passed to a separator wherein the pressure is released, causing the solvent to vaporize, leaving the liquid oil for removal. After recompression, the solvent vapors are recycled to solvent storage.
Removal of solvent by vaporization consumes large amount of heat because of the absorption of the heat of vaporization which is not recoverable for further vaporization although it can, of course, be used for raising the sensible heat of fresh or recycled solvent. Also, the specific heat of, e.g., propane, increases drastically as its critical temperature is approached. The first of these problems is solved (while the second is actually aggravated by U.S. Pat. No. 4,331,695 wherein batchwise extraction and thermal phase separation of the miscella are carried out isobarically at near-critical pressure, the temperature of the extraction being near critical and that of the phase separation being super-critical without taking up the heat of vaporization. At super-critical temperature, a dense phase of oil separates and can be removed while a less dense phase of solvent can be recirculated by a pump to the extraction, giving up heat to miscella proceeding to phase separation and becoming more dense for greater solvent action.
Another suggested approach has been to admix the raw material with a sufficient amount of oil to form a pumpable slurry which lends itself to a continuous extraction operation since a slurry can be continuously introduced into the pressurized extraction unit under a greater pump pressure using available high pressure pumps. One such disclosure appears in U.S. Pat. No. 2,548,434 wherein the extraction unit for vegetable oils, such as soybean and cottonseed oils, using propane or the like, in countercurrent flow is maintained at near critical temperature, usually 150.degree.-250.degree. C., and under a sufficiently high pressure to maintain the solvent liquid, e.g., from about 400-600 pounds. At these conditions and with a ratio of solvent to extracted oil of 15-30:1, the extraction proceeds selectively or preferentially for the light colored fatty matter, while undesirable components including color bodies, phosphotides, gums, etc., are rejected and remain with the residue, e.g., as a heavy miscella phase. The solid residue and the latter phase can be withdrawn from the unit and the solvent flashed off under a vacuum flasher. The lighter miscella leaves from the top of the unit and may also be subjected to flashing to remove the solvent and leave the liquid oil for recovery, the flasher temperature being reduced to around 120.degree. F. The liquid oil from the flasher can be chilled in one or more stages under still lower pressure and a sufficiently low temperature to crystalize a high melting oil fraction which is removed by filtration and stripped of residual solvent. The remaining lower-melting fraction is stripped in a three-stage unit in which the initial stage operates at a pressure exceeding the extraction pressure and the final stage under a vacuum.
Handling of the raw solids in pumpable slurry form with liquified solvent is also taught in U.S. Pat. No. 2,564,409, using countercurrent extraction flow at a temperature from ambient to 160.degree. F., a pressure of 50-700 psi, and a solvent/oil ratio by volume of 3-10:1. The residual solids are removed as a slurry, and the solvent is flashed under reduced pressure with heating. The miscella is subjected to fractionation in countercurrent contact with fresh upflowing solvent at approximately its critical temperature, e.g., about 150.degree.-210.degree. F., at which the extracted oil loses solubility in the solvent, the temperature being adjusted to select the desired fraction. A part of the miscella can be separated before fractionation and, after solvent stripping, recycled for slurrying with fresh raw material.
While slurries of the raw material in the liquified solvent are suitable for continuous processing, they require large excess quantities of solvent over that needed merely for efficient extraction of the oil content of the raw material. Such excess quantities in turn have to be separated from the oil and treated solids which greatly increase the amounts of heat (or cooling) required and of power necessary to recompress (or evacuate) the solvent to effect the changes in the state of the solvent. Thus, the overall consumption of energy inherent in slurry operation is at least non-competitive, if not prohibitive. Nonetheless, continuous operation is virtually indispensable for the economical processing of large volumes of raw material for marketing at the low cost price that can be demanded for these bulk commodities, and isolated attempts are found in the art to carry out some kind of continuous processing with liquified solvents that does not depend upon the use of solvent slurries.
An early example, as an optional alternative to straight batch operation, is found in U.S. Pat. No. 2,247,851 which is directed to the extraction of the residual oil content of so-called cracklings, i.e., the brittle solid sediment resulting from the steam rendering of lard from animal fats to yield a protein-containing solid suitable for chicken feed. Here, the cracklings were extracted in a horizontal chamber housing several axially arranged rotating drums in contact with liquified solvent flowing at a certain depth therethrough, being loaded and emptied at opposite ends of the chamber by horizontal screw conveyors, the downstream one of which was steam-heated. Each such conveyor was, in turn, equipped with a double-lock compartment to admit or discharge the material in successive batches in an approximation of continuous operation. The extracted solids were removed from the chamber, digested in admixture with hot water and finally dried.
The same concept was adapted in U.S. Pat. No. 2,254,245 to cottonseed meats to produce cottonseed oil of improved quality using multiple serially-joined extraction stages, the series being preceded and followed by the loading and unloading screw conveyors and associated feeding compartments, the miscella of each stage being separately manipulated during purification. The first extraction stage was held at -5 to -15.degree. F. to exclude high melting components from the extracted oil and minimize the amount of dissolved coloring matter, with subsequent stages at progressively higher temperatures up to, e.g., about 80.degree. F. Conditions for separating the miscella components were also controlled, the first-stage miscella after filtration being distilled initially at no more than 210.degree. F. under super-atmospheric pressure with the remaining liquid phase re-distilled at progressively decreasing pressures. Distillation of the miscella from the other stages took place similarly.
Quite recently, continuous extraction was described in EP application 0129739, published Jan. 2, 1985, where a vertical extraction vessel was fed by means of a screw-press compressing the oil-containing raw material under sufficient mechanical pressure to express directly a substantial amount of oil, e.g., 50% by weight, which was removed through a perforated wall. This press then, by means of a throttling constriction in its annular area downstream of such oil expression and just upstream of its connection into the extraction vessel created a constantly regenerated solid plug or dam of raw material resisting the extraction pressure. Extraction was under super-critical conditions with CO.sub.2, propane or the like, under a pressure ranging from 250-750 bar (about 3600-10880 psi), at a temperature of 40.degree.-110.degree. C., and for a treatment time of 0.5-2.5 hours. Pressure at the outlet end of the vessel was maintained by a similar non-expressing screw press creating a similar solid plug in the same manner. The extracted miscella was separated into multiple fractions by multi-stage fractionation at different combinations of heat and reduced pressure.
The power required to drive a high compression screw-press as in the latter system, especially if and for cereal brans, is extremely large and, moreover, mechanically expressed oil tends to be of low quality with an unacceptable level of impurities and must be further purified. Hence, the need for an energy-efficient and economical continuous procedure for extracting oils from natural vegetable matter, particularly cereal brans, with a low boiling liquified solvent remains unsatisfied in the art.
The extraction of oil from brown rice, and rice bran specifically (as here preferred), with normally liquid solvents is itself well known in this art. In U.S. Pat. No. 2,538,007, brown rice is said to be stabilized against its normal susceptability toward rancidity due to lipolysis by extracting the same with hexane or like normally liquid solvent at a temperature from ambient to the solvent boiling point to remove "free oil", the principal cause of rancidity in rice.
The oil content of rice is concentrated in the germ and bran, i.e., the brown layer beneath the hull, and extraction of oil from the bran was found, according to U.S. Pat. No. 2,727,914, to be promoted by subjecting the bran to mild pre-cooking at 170.degree.-235.degree. F., with its moisture content controlled at 14-26%, for 15-70 minutes preferably in several stages of increasing temperature and decreasing moisture content. Reduced solvent/raw material ratios of 0.7-1/1 (about one-half the normal requirements absent pre-cooking) were effective.
In U.S. Pat. No. 2,829,055, rice oil is obtained by extracting whole dehulled rice grains still carrying their brown covering or bran, with hexane, etc., at a temperature below its boiling point, e.g., 40.degree.-60.degree. C. Such treatment renders the bran more readily removable from the rice grains by subsequent milling. Rice oil is separated from the solvent by fractional distillation.
The actual milling of brown rice, according to U.S. Pat. No. 3,261,690, is carried out in the presence of solvent, preferably hexane, for simultaneous extraction of the fatty components, the bran being softened and flushed away by the solvent which also minimizes overheating and prevents damage to the rice grains. The oil and solvent, after removal of the bran solids, are separated by an evaporator separator and stripping column. Preferably, recovered rice oil or a strong miscella thereof is recycled for admixture with the brown rice as a bran-softening agent.